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Statistics LibreTexts

1.1: Descriptive Statistics

  • Page ID
    2869
  • A population is the group to be studied, and population data is a collection of all elements in the population. For example:

    • All the fish in Long Lake.
    • All the lakes in the Adirondack Park.
    • All the grizzly bears in Yellowstone National Park.

    A sample is a subset of data drawn from the population of interest. For example:

    • 100 fish randomly sampled from Long Lake.
    • 25 lakes randomly selected from the Adirondack Park.
    • 60 grizzly bears with a home range in Yellowstone National Park.

    Image35759.PNG

    Figure 1. Using sample statistics to estimate population parameters. 

    Populations are characterized by descriptive measures called parameters. Inferences about parameters are based on sample statistics. For example, the population mean (\(μ\)) is estimated by the sample mean (\( \bar x\) ). The population variance ( \(\sigma ^2\)) is estimated by the sample variance (\(s^2\)).

    Variables are the characteristics we are interested in. For example:

    • The length of fish in Long Lake.  
    • The pH of lakes in the Adirondack Park.
    • The weight of grizzly bears in Yellowstone National Park.

    Variables are divided into two major groups: qualitative and quantitative. Qualitative variables have values that are attributes or categories. Mathematical operations cannot be applied to qualitative variables. Examples of qualitative variables are gender, race, and petal color. Quantitative variables have values that are typically numeric, such as measurements. Mathematical operations can be applied to these data. Examples of quantitative variables are age, height, and length. Quantitative variables can be broken down further into two more categories: discrete and continuous variables. Discrete variables have a finite or countable number of possible values. Think of discrete variables as “hens”. Hens can lay 1 egg, or 2 eggs, or 13 eggs… There are a limited, definable number of values that the variable could take on.

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    Continuous variables have an infinite number of possible values. Think of continuous variables as “cows”. Cows can give 4.6713245 gallons of milk, or 7.0918754 gallons of milk, or 13.272698 gallons of milk … There are an almost infinite number of values that a continuous variable could take on.

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    Example \(\PageIndex{1}\):

    Is the variable qualitative or quantitative?

    1. Species
    2. Weight
    3. Diameter
    4.  Zip Code

    Solution

    (qualitative quantitative, quantitative, qualitative)

    Descriptive Measures

    Descriptive measures of populations are called parameters and are typically written using Greek letters. The population mean is \(\mu\) (mu). The population variance is \(\sigma ^2\) (sigma squared) and population standard deviation is \(\sigma \) (sigma). Descriptive measures of samples are called statistics and are typically written using Roman letters. The sample mean is \(\bar x\) (x-bar). The sample variance is \(s^2\) and the sample standard deviation is \(s\). Sample statistics are used to estimate unknown population parameters. In this section, we will examine descriptive statistics in terms of measures of center and measures of dispersion. These descriptive statistics help us to identify the center and spread of the data.

    Measures of Center

    Mean

    The arithmetic mean of a variable, often called the average, is computed by adding up all the values and dividing by the total number of values. The population mean is represented by the Greek letter \(\mu\) (mu). The sample mean is represented by \(\bar x\) (x-bar). The sample mean is usually the best, unbiased estimate of the population mean. However, the mean is influenced by extreme values (outliers) and may not be the best measure of center with strongly skewed data. The following equations compute the population mean and sample mean.

    $$\mu = \frac {\sum x_i}{N}$$

    $$\bar x = \frac {\sum x_i}{n}$$

    where \(x_i\) is an element in the data set, \(N\) is the number of elements in the population, and \(n\) is the number of elements in the sample data set.

    Example \(\PageIndex{2}\): mean

    Find the mean for the following sample data set:

    6.4, 5.2, 7.9, 3.4

    Solution

    $$\bar x = \frac {6.4+5.2+7.9+3.4} {4} = 5.725$$

    Median

    The median of a variable is the middle value of the data set when the data are sorted in order from least to greatest. It splits the data into two equal halves with 50% of the data below the median and 50% above the median. The median is resistant to the influence of outliers, and may be a better measure of center with strongly skewed data.

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    The calculation of the median depends on the number of observations in the data set.

    To calculate the median with an odd number of values (n is odd), first sort the data from smallest to largest.

    Example \(\PageIndex{3}\): Calculating Median with Odd number of values

    Find the median for the following sample data set:

    $$23, 27, 29, 31, 35, 39, 40, 42, 44, 47, 51$$

    Solution

    The median is 39. It is the middle value that separates the lower 50% of the data from the upper 50% of the data.

    To calculate the median with an even number of values (n is even), first sort the data from smallest to largest and take the average of the two middle values.

    Example \(\PageIndex{4}\): Calculating Median with even number of values

    Find the median for the following sample data set:

    $$23, 27, 29, 31, 35, 39, 40, 42, 44, 47$$

    Solution

    $$M = \frac {35+39}{2} = 37$$

    Mode

    The mode is the most frequently occurring value and is commonly used with qualitative data as the values are categorical. Categorical data cannot be added, subtracted, multiplied or divided, so the mean and median cannot be computed. The mode is less commonly used with quantitative data as a measure of center. Sometimes each value occurs only once and the mode will not be meaningful.

    Understanding the relationship between the mean and median is important. It gives us insight into the distribution of the variable. For example, if the distribution is skewed right (positively skewed), the mean will increase to account for the few larger observations that pull the distribution to the right. The median will be less affected by these extreme large values, so in this situation, the mean will be larger than the median. In a symmetric distribution, the mean, median, and mode will all be similar in value. If the distribution is skewed left (negatively skewed), the mean will decrease to account for the few smaller observations that pull the distribution to the left. Again, the median will be less affected by these extreme small observations, and in this situation, the mean will be less than the median.

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    Figure 2. Illustration of skewed and symmetric distributions.

    Measures of Dispersion

    Measures of center look at the average or middle values of a data set. Measures of dispersion look at the spread or variation of the data. Variation refers to the amount that the values vary among themselves. Values in a data set that are relatively close to each other have lower measures of variation. Values that are spread farther apart have higher measures of variation.

    Examine the two histograms below. Both groups have the same mean weight, but the values of Group A are more spread out compared to the values in Group B. Both groups have an average weight of 267 lb. but the weights of Group A are more variable.

    860.png

    Figure 3. Histograms of Group A and Group B.

    This section will examine five measures of dispersion: range, variance, standard deviation, standard error, and coefficient of variation.

    Range

    The range of a variable is the largest value minus the smallest value. It is the simplest measure and uses only these two values in a quantitative data set.

    Example \(\PageIndex{5}\): Computing Range

    Find the range for the given data set.

    $$12, 29, 32, 34, 38, 49, 57$$

    $$Range = 57 – 12 = 45$$

    Variance

    The variance uses the difference between each value and its arithmetic mean. The differences are squared to deal with positive and negative differences. The sample variance (\(s^2\)) is an unbiased estimator of the population variance (\(\sigma ^2\)), with n-1 degrees of freedom.

    Degrees of freedom: In general, the degrees of freedom for an estimate is equal to the number of values minus the number of parameters estimated en route to the estimate in question.

    The sample variance is unbiased due to the difference in the denominator. If we used “n” in the denominator instead of “n – 1”, we would consistently underestimate the true population variance. To correct this bias, the denominator is modified to “n – 1”.

    Definition: population variance

    $$\sigma ^2 = \frac {\sum (x_i-\mu)^2} {N}$$

    Definition: sample variance

    $$ s^2 = \frac {\sum (x_i- \bar x)^2}{n-1} = \frac {\sum x_i^2 - \frac {(\sum x_i)^2}{n}}{n-1} \label{samplevar}$$

    Example \(\PageIndex{6}\): Computing Variance

    Compute the variance of the sample data: 3, 5, 7.

    Solution

    The sample mean (\( \bar x\)) is 5. Then use Equation \ref{samplevar}

    $$s^2 = \frac {(3-5)^2 +(5-5)^2 + (7-5) ^2} {3-1} = 4$$

    Standard Deviation 

    The standard deviation is the square root of the variance (both population and sample). While the sample variance is the positive, unbiased estimator for the population variance, the units for the variance are squared. The standard deviation is a common method for numerically describing the distribution of a variable. The population standard deviation is σ (sigma) and sample standard deviation is s.

    Definition:SAMPLE STANDARD DEVIATION

    $$s = \sqrt {s^2}$$

    Definition:POPULATION STANDARD DEVIATION

    $$\sigma = \sqrt {\sigma ^2}$$

    Example \(\PageIndex{7}\):

    Compute the standard deviation of the sample data: 3, 5, 7 with a sample mean of 5.

    Solution

    The sample mean (\(\bar x\)) is 5, using the definition of standard deviation

    $$s = \sqrt {\frac {(3-5)^2+(5-5)^2+(7-5)^2} {3-1}} = \sqrt {4} = 2$$

    Standard Error of Mean

    Commonly, we use the sample mean  to estimate the population mean μ. For example, if we want to estimate the heights of eighty-year-old cherry trees, we can proceed as follows:

    • Randomly select 100 trees
    • Compute the sample mean of the 100 heights
    • Use that as our estimate

    We want to use this sample mean to estimate the true but unknown population mean. But our sample of 100 trees is just one of many possible samples (of the same size) that could have been randomly selected. Imagine if we take a series of different random samples from the same population and all the same size:

    • Sample 1—we compute sample mean \(\bar x\)
    • Sample 2—we compute sample mean \(\bar x\)
    • Sample 3—we compute sample mean \(\bar x\)
    • Etc.

    Each time we sample, we may get a different result as we are using a different subset of data to compute the sample mean. This shows us that the sample mean is a random variable!

    The sample mean (\(\bar x\)) is a random variable with its own probability distribution called the sampling distribution of the sample mean. The distribution of the sample mean will have a mean equal to µ and a standard deviation equal to \(\frac {s} {\sqrt {n}}\)

    Note

    The standard error \(\frac {s} {\sqrt {n}}\) is the standard deviation of all possible sample means.

    In reality, we would only take one sample, but we need to understand and quantify the sample to sample variability that occurs in the sampling process.

    The standard error is the standard deviation of the sample means and can be expressed in different ways.

    $$s_{\bar x}=\sqrt {\frac {s^2}{n}}=\frac {s}{\sqrt {n}}$$

    Note

    \(s^2\) is the sample variance and s is the sample standard deviation

    Example \(\PageIndex{8}\):

    Describe the distribution of the sample mean.

    A population of fish has weights that are normally distributed with µ = 8 lb. and s = 2.6 lb. If you take a sample of size n=6, the sample mean will have a normal distribution with a mean of 8 and a standard deviation (standard error) of \(\frac {2.6}{\sqrt {6}}\)= 1.061 lb.

    If you increase the sample size to 10, the sample mean will be normally distributed with a mean of 8 lb. and a standard deviation (standard error) of \(\frac {2.6}{\sqrt {10}}\) = 0.822 lb.

    Notice how the standard error decreases as the sample size increases.

    The Central Limit Theorem (CLT) states that the sampling distribution of the sample means will approach a normal distribution as the sample size increases. If we do not have a normal distribution, or know nothing about our distribution of our random variable, the CLT tells us that the distribution of the ’s will become normal as nincreases. How large does n have to be? A general rule of thumb tells us that n ≥ 30.

    Note

    The Central Limit Theorem tells us that regardless of the shape of our population, the sampling distribution of the sample mean will be normal as the sample size increases.

    Coefficient of Variation 

    To compare standard deviations between different populations or samples is difficult because the standard deviation depends on units of measure. The coefficient of variation expresses the standard deviation as a percentage of the sample or population mean. It is a unitless measure.

    Definition: CV of Population

    $$CV=\frac {\sigma}{\mu} \times 100$$

    Definition: cv of sample

    $$CV=\frac {s}{\bar x} \times 100$$

    Example \(\PageIndex{9}\):

    Fisheries biologists were studying the length and weight of Pacific salmon. They took a random sample and computed the mean and standard deviation for length and weight (given below). While the standard deviations are similar, the differences in units between lengths and weights make it difficult to compare the variability. Computing the coefficient of variation for each variable allows the biologists to determine which variable has the greater standard deviation.

      Sample mean Sample standard deviation
    Length 63 cm 19.97 cm
    Weight 37.6 kg 19.39 kg
      685.png  678.png 

    There is greater variability in Pacific salmon weight compared to length.

    Variability  

    Variability is described in many different ways. Standard deviation measures point to point variability within a sample, i.e., variation among individual sampling units. Coefficient of variation also measures point to point variability but on a relative basis (relative to the mean), and is not influenced by measurement units. Standard error measures the sample to sample variability, i.e. variation among repeated samples in the sampling process. Typically, we only have one sample and standard error allows us to quantify the uncertainty in our sampling process.

    Basic Statistics Example using Excel and Minitab Software

    Consider the following tally from 11 sample plots on Heiburg Forest, where Xi is the number of downed logs per acre. Compute basic statistics for the sample plots.

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    Table 1. Sample data on number of downed logs per acre from Heiburg Forest.

    (1) Sample mean: 654.png

    (2) Median = 35

    (3) Variance:

    644.png

    (4) Standard deviation:  634.png

    (5) Range: 55 – 5 = 50

    (6) Coefficient of variation:

    625.png

    (7) Standard error of the mean:

    618.png

    Software Solutions

    Minitab

    Open Minitab and enter data in the spreadsheet. Select STAT>Descriptive stats and check all statistics required.

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    Descriptive Statistics: Data

    Variable

    N

    N*

    Mean

    SE Mean

    StDev

    Variance

    CoefVar

    Minimum

    Q1

    Data

    11

    0

    32.27

    4.83

    16.03

    256.82

    49.66

    5.00

    20.00

    Variable

    Median

    Q3

    Maximum

    IQR

    Data

    35.00

    45.00

    55.00

    25.00

    Excel

    Open up Excel and enter the data in the first column of the spreadsheet. Select DATA>Data Analysis>Descriptive Statistics. For the Input Range, select data in column A. Check “Labels in First Row” and “Summary Statistics”. Also check “Output Range” and select location for output.

    009_2.tif

    009_1.tif

    Data

    Mean

    32.27273

    Standard Error

    4.831884

    Median

    35

    Mode

    25

    Standard Deviation

    16.02555

    Sample Variance

    256.8182

    Kurtosis

    -0.73643

    Skewness

    -0.05982

    Range

    50

    Minimum

    5

    Maximum

    55

    Sum

    355

    Count

    11

     Graphic Representation 

    Data organization and summarization can be done graphically, as well as numerically. Tables and graphs allow for a quick overview of the information collected and support the presentation of the data used in the project. While there are a multitude of available graphics, this chapter will focus on a specific few commonly used tools.

    Pie Charts

    Pie charts are a good visual tool allowing the reader to quickly see the relationship between categories. It is important to clearly label each category, and adding the frequency or relative frequency is often helpful. However, too many categories can be confusing. Be careful of putting too much information in a pie chart. The first pie chart gives a clear idea of the representation of fish types relative to the whole sample. The second pie chart is more difficult to interpret, with too many categories. It is important to select the best graphic when presenting the information to the reader.

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    Figure 4. Comparison of pie charts.

     

    Bar Charts and Histograms

    Bar charts graphically describe the distribution of a qualitative variable (fish type) while histograms describe the distribution of a quantitative variable discrete or continuous variables (bear weight).

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    Figure 5. Comparison of a bar chart for qualitative data and a histogram for quantitative data.

     

    In both cases, the bars’ equal width and the y-axis are clearly defined. With qualitative data, each category is represented by a specific bar. With continuous data, lower and upper class limits must be defined with equal class widths. There should be no gaps between classes and each observation should fall into one, and only one, class.

    Boxplots

    Boxplots use the 5-number summary (minimum and maximum values with the three quartiles) to illustrate the center, spread, and distribution of your data. When paired with histograms, they give an excellent description, both numerically and graphically, of the data.

    With symmetric data, the distribution is bell-shaped and somewhat symmetric. In the boxplot, we see that Q1 and Q3 are approximately equidistant from the median, as are the minimum and maximum values. Also, both whiskers (lines extending from the boxes) are approximately equal in length.

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    012_1.tif

     

     

    Figure 6. A histogram and boxplot of a normal distribution.

     

    With skewed left distributions, we see that the histogram looks “pulled” to the left. In the boxplot, Q1 is farther away from the median as are the minimum values, and the left whisker is longer than the right whisker.

    013_2.tif

     

     

    Figure 7. A histogram and boxplot of a skewed left distribution.

     

    013_1.tif

    With skewed right distributions, we see that the histogram looks “pulled” to the right. In the boxplot, Q3 is farther away from the median, as is the maximum value, and the right whisker is longer than the left whisker.

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    014_1.tif

     

     

    Figure 8. A histogram and boxplot of a skewed right distribution.